Battery Charging

[labatt]

OK... lots of battery questions for me these days!

We have two 8D AGM's (490AH). We only drain about 150AH to 180AH out of them before we recharge. The most I've ever drained them is 235AH. We charge them through a generator powered Xantrex Inverter/Charger, and monitor the charge on a Xantrex Link 1000. A few weeks ago, when they were drained, we'd see the charge rate at about 60-65amps until they were about 85% charged, and then would see the number dwindle down to about a 25-30 amp charge rate (i.e. they appeared to be charging correctly). Over the past few days, I've noticed that when they are about 70% charged, the charge rate starts "dancing" between 30amps and 50amps - changing every few seconds. There has been no load change on our electrical system (that I'm aware of) during the charging time.

Since there has been a change in the way the batteries charge, I'm trying to figure out if the batteries have started living out their useful lifetime (they are about 5 years old, and now we're liveaboards so charge every day), or if perhaps the charger is starting to fail. Any thoughts on where I should start looking? I'm new to this battery stuff!

[chucklesR]

Sounds like it might be the batteries getting older but you have one more variable that you haven't mentioned that has significant impact on battery charging.

You are now in a warmer climate.

Temperature does matter. Do you have a temp monitor on your battery charging equipment?

[xort]

Have you equalized?
Have you topped off to 100% with shore power recently.
Might need to top off overnight and then EQ.
How does the specific gravity look? Even across all cells?

[labatt]

They are sealed AGM's, so generally you don't have to equalize (from what I understand) and there's no way to measure the specific gravity (as far as I know). We haven't topped off with shore power, but we topped off with generator power a couple of weeks ago. Normally, we bring them to 90%-95% with the generator. Now I'm having trouble even bringing them to 80%.

[labatt]

Chuckles - I have to verify if we have a temp sensor. I believe we do. The batteries and the charger, however, are below water line and the air temp around them is generally around 70-75 degrees.

[xort]

correct, no EQ for AGM

Check the resting voltage about a half hour after charging. See if it's where it should be. Resting voltage is with no drain

[labatt]

I don't think I'm going to get anywhere near 90% charge today. They've been charging for 2 hours and are only at 75%. Always fun...

[chucklesR]

If you had a temp probe I'd think you'd know.
Rechargeable Batteries Applications ... - Google Book Search
shows the effect of temperature.
The air temp is room temp, what's the battery temp? Especially now that the water is warmer etc..
I don't mean to sing a single tune, but it fits the music

[Vasco]

Vero has been known to suck the life out of everything, including batteries. On a more serious note your batteries have not been getting a full charge lately as you have to run the charger a very long time to get them right up. I have found that on the hook you end up using the band between 50 and 85% as it just takes too much to bring them up tp 100%. Over time your capacity decreases, I have found Links to be less than accurate (it may just be me). As to the charge fluctuating so greatly have you checked all your connections?

[Izzy1414]

labatt,

I'll throw my $.02 in from my 30+ years in dealing with equipment (albiet not all marine) with group batteries and their maintenance. First, Chuckles is absolutely right, changes in temperature and usage will affect charging system and battery performance. Problems that didn't show themselves before will crop up when you change the environment where they operate. Whenever I encounter an electrical problem (or any problem for that matter) I try to eliminate the easy things first. In my experience, a vast majority of battery charging problems have bad or corroded connections as at least a contributing cause. Not always, but the best place to start. Next, I'll check to see if the monitoring guages on the equipment is telling me what is really happening. A good volt meter will tell you a lot. Take static readings on each battery and then readings while you are charging. These will help check the accuracy of your on-boat guages as well as compare the relative condition of each battery. If one battery shows a significant difference from the others I would suspect a battery problem. Especially if the group has been equalized. The next step would be to check the condition of each battery. The only way I know of to that with your type of sealed batteries is with a carbon-pile load tester. It tests the battery's ability to draw a load and sustain a minimum voltage. The problem is that a good quality tester is quite expensive so your best bet is to take your batteries out and to a shop that has one. Remember that the battery must be fully charged to get accurate test results. One last thing I can think of on the easier side of the ledger...make sure that you don't have something on your boat that is drawing a higher load than it used to. Especially with isolated batteries a changed load may affect one battery differently than another.